Continuous fiber reinforced composites and methods, apparatuses, and compositions for making the same

ABSTRACT

A process for continuous composite coextrusion comprising: (a) forming first a material-laden composition comprising a thermoplastic polymer and at least about 40 volume % of a ceramic or metallic particulate in a manner such that the composition has a substantially cylindrical geometry and thus can be used as a substantially cylindrical feed rod; (b) forming a hole down the symmetrical axis of the feed rod; (c) inserting the start of a continuous spool of ceramic fiber, metal fiber or carbon fiber through the hole in the feed rod; (d) extruding the feed rod and spool simultaneously to form a continuous filament consisting of a green matrix material completely surrounding a dense fiber reinforcement and said filament having an average diameter that is less than the average diameter of the feed rod; and (e) depositing the continuous filament into a desired architecture which preferably is determined from specific loading conditions of the desired object and CAD design of the object to provide a green fiber reinforced composite object.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/038,957, filed on Jan. 2, 2002, which claims the benefit of U.S.Provisional Application Ser. No. 60/259,284, filed on Jan. 2, 2001, andentitled “Automated Tow Placement Process for Fabricating FiberReinforced Ceramic Matrix Composites”. These applications areincorporated herein by reference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to continuous composite coextrusionmethods, apparatuses for coextrusion, and compositions for preparingcomposites, such as continuous fiber reinforced ceramic matrixcomposites, using dense fibers and green matrices as well as to methodsand apparatuses for the preparation of composites having interfacesbetween dense fibers and green matrices, particularly three-dimensionalobjects having complex geometries.

BACKGROUND OF THE INVENTION

Composites are combinations of two or more materials present as separatephases and combined to form desired structures so as to take advantageof certain desirable properties of each component. The materials can beorganic, inorganic, or metallic, and in various forms, including but notlimited to particles, rods, fibers, plates and foams. Thus, a composite,as defined herein, although made up of other materials, can beconsidered to be a new material have characteristic properties that arederived from its constituents, from its processing, and from itsmicrostructure.

Composites are made up of the continuous matrix phase in which areembedded: (1) a three-dimensional distribution of randomly orientedreinforcing elements, e.g., a particulate-filled composite; (2) atwo-dimensional distribution of randomly oriented elements, e.g., achopped fiber mat; (3) an ordered two-dimensional structure of highsymmetry in the plane of the structure, e.g., an impregnated clothstructure; or (4) a highly-aligned array of parallel fibers randomlydistributed normal to the fiber directions, e.g., a filament-woundstructure, or a prepreg sheet consisting of parallel rows of fibersimpregnated with a matrix.

Monolithic ceramic materials are known to exhibit certain desirableproperties, including high strength and high stiffness at elevatedtemperatures, resistance to chemical and environmental attack, and lowdensity. However, monolithic ceramics have one property that limitstheir use in stressed environments, namely their low fracture toughness.While significant advances have been made to improve the fracturetoughness of monolithic ceramics, mostly through the additions ofwhisker and particulate reinforcements or through careful control of themicrostructural morphology, they still remain extremely damageintolerant. More specifically, they are susceptible to thermal shock andwill fail catastrophically when placed in severe stress applications.Even a small processing flaw or crack that develops in a stressedceramic cannot redistribute or shed its load on a local scale. Underhigh stress or even mild fatigue, the crack will propagate rapidlyresulting in catastrophic failure of the part in which it resides. It isthis inherently brittle characteristic which can be even more pronouncedat elevated temperatures, that has not allowed monolithic ceramics to beutilized in any safety-critical designs.

Research and development for these high temperature and high stressapplications have focused on the development of continuous fiberreinforced ceramic matrix composites, hereafter referred to as CFCCs.The use of fiber reinforcements in the processing of ceramic and metalmatrix composites is known in the prior art, and has essentiallyprovided the fracture toughness necessary for ceramic materials to bedeveloped for high stress, high temperature applications. See J. J.Brennan and K. M. Prewo, “High Strength Silicon Carbide Fiber ReinforcedGlass-Matrix Composites,” J. Mater. Sci., 15 463-68 (1980); J. J.Brennan and K. M. Prewo, “Silicon Carbide Fiber Reinforced Glass-CeramicMatrix Composites Exhibiting High Strength Toughness,” J. Mater. Sci.,17 2371-83 (1982); P. Lamicq, G. A. Gernhart, M. M. Danchier, and J. G.Mace, “SiC/SiC Composite Ceramics,” Am. Ceram. Soc. Bull., 65 [2] 336-38(1986); T. I. Mah, M. G. Mendiratta, A. P. Katz, and K. S. Mazdiyasni,“Recent Developments in Fiber-Reinforced High Temperature CeramicComposites,” Am. Ceram. Soc. Bull., 66 [2] 304-08 (1987); K. M. Prewo,“Fiber-Reinforced Ceramics: New Opportunities for Composite Materials,”Am. Ceram. Soc. Bull., 68 [2] 395-400 (1989); H. Kodama, H. Sakamoto,and T. Miyoshi, “Silicon Carbide Monofilament-Reinforced Silicon Nitrideor Silicon Carbide Matrix Composites,” J. Am. Ceram. Soc., 72 [4] 551-58(1989); and J. R. Strife, J. J. Brennan, and K. M. Prewo, “Status ofContinuous Fiber-Reinforced Ceramic Matrix Composite ProcessingTechnology,” Ceram. Eng. Sci. Proc., 11 [7-8] 871-919 (1990).

Under high stress conditions, the fibers are strong enough to bridge thecracks which form in the ceramic matrix allowing the fibers toultimately carry the load, and catastrophic failure can be avoided. Thistype of behavior has led to a resurgence of CFCCs as potential materialsfor gas turbine components, such as combustors, first-stage vanes, andexhaust flaps. See D. R. Dryell and C. W. Freeman, “Trends in Design inTurbines for Aero Engines,” pp. 38-45 in Materials Development inTurbo-Machinery Design; 2nd Parsons International Turbine Conference,Edited by D. M. R. Taplin, J. F. Knott, and M. H. Lewis, The Instituteof Metals, Parsons Press, Trinity College, Dublin, Ireland, 1989. CFCCshave also been given serious consideration for heat exchangers, rocketnozzles, and the leading edges of next-generation aircraft and reentryvehicles. See M. A. Karnitz, D. F. Craig, and S. L. Richlin, “ContinuousFiber Ceramic Composite Program,” Am. Ceram. Soc. Bull., 70 [3] 430-35(1991), and Flight Vehicle Materials, Structures and Dynamics—Assessmentand Future Directions, Vol. 3, edited by S. R. Levine, American Societyof Mechanical Engineers, New York, 1992. In addition, CFCCs with a highlevel of open porosity are currently being utilized as filters forhot-gas cleanup in electrical power generation systems, metal refining,chemical processing, and diesel exhaust applications. See L. R. White,T. L. Tompkins, K. C. Hsieh, and D. D. Johnson, “Ceramic Filters for HotGas Cleanup,” J. Eng. for Gas Turbines and Power, Vol. 115, 665-69(1993).

CFCCs are currently fabricated by a number of techniques. The simplestand most common method for their fabricating has been the slurryinfiltration technique whereby a fiber or fiber tow is passed through aslurry containing the matrix powder; the coated fiber is then filamentwound to create a “prepreg”; the prepreg is removed, cut, oriented, andlaminated into a component shape; and the part undergoes binderpyrolysis and a subsequent firing cycle to densify the matrix. See J. J.Brennan and K. M. Prewo, “High Strength Silicon Carbide Fibre ReinforcedGlass-Matrix Composites,” J. Mater. Sci., 15 463-68 (1980); D. C.Phillips, “Fiber Reinforced Ceramics,” Chapter 7 in Fabrication ofComposites, edited by A. Kelly and S. T. Mileiko, North-HollandPublishing Company, Amsterdam, The Netherlands, 1983; and K. M. Prewoand J. J. Brennan, “Silicon Carbide Yarn Reinforced Glass MatrixComposites,” J. Mater. Sci., 17 1201-06 (1982).

Other techniques for fabricating CFCCs also typically involve aninfiltration process in order to incorporate matrix material within andaround the fiber architecture, e.g. a fiber tow, a preformed fiber mat,a stack of a plurality of fiber mats, or other two dimensional (2D) orthree dimensional (3D) preformed fiber architecture. These techniquesinclude the infiltration of sol-gels. See J. J. Lannutti and D. E.Clark, “Long Fiber Reinforced Sol-Gel Derived Alumina Composites”, pp.375-81 in Better Ceramics Through Chemistry, Material Research SocietySymposium Proceedings, Vol. 32, North-Holland, New York, 1984; E. Fitzerand R. Gadow, “Fiber Reinforced Composites Via the Sol-Gel Route”, pp.571-608 in Tailoring Multiphase and Composite Ceramics, MaterialsScience Research Symposium Proceedings, Vol. 20, edited by R. E.Tressler et al., Plenum Press, New York, 1986. Other techniques includepolymeric precursors which are converted to the desired ceramic matrixmaterial through a post-processing heat treatment. See J. Jamet, J. R.Spann, R. W. Rice, D. Lewis, and W. S. Coblenz, “Ceramic-Fiber CompositeProcessing via Polymer-Filler Matrices,” Ceram. Eng. Sci. Proc., 5 [7-8]677-94 (1984); and K. Sato, T. Suzuki, O. Funayama, T. Isoda,“Preparation of Carbon Fiber Reinforced Composite by Impregnation withPerhydropolysilazane Followed by Pressureless Firing,” Ceram. Eng. Sci.Proc., 13 [9-10] 614-21 (1992).

Other research and development has involved molten metals that are laternitrided or oxidized. See M. S. Newkirk, A. W. Urquhart, H. R. Zwicker,and E. Breval, “Formation of Lanxide Ceramic Composite Materials,” J.Mater. Res., 1 81-89 (1986); and M. K. Aghajanian, M. A. Rocazella, J.T. Burke, and S. D. Keck, “The Fabrication of Metal Matrix Composites bya Pressureless Infiltration Technique,” J. Mater. Sci., 26 447-54(1991). Other research and development has involved molten materialsthat are later carbided to form a ceramic matrix. See R. L. Mehan, W. B.Hillig, and C. R. Morelock, “Si/SiC Ceramic Composites: Properties andApplications,” Ceram. Eng. Sci. Proc., 1 405 (1980). Still otherresearch and development has involved molten silicates that cool to forma glass or glass-ceramic matrix (see M. K. Brun, W. B. Hillig, and H. C.McGuigan, “High Temperature Mechanical Properties of a ContinuousFiber-Reinforced Composite Made by Melt Infiltration,” Ceram. Eng. Sci.Proc., 10 [7-8] 611-21 (1989)), and chemical vapors which decompose andcondense to form the ceramic matrix (See A. J. Caputo and W. J. Lackey,“Fabrication of Fiber-Reinforced Ceramic Composites by Chemical VaporInfiltration,” Ceram. Eng. Sci. Proc., 5 [7-8] 654-67 (1984); and A. J.Caputo, W. J. Lackey, and D. P. Stinton, “Development of a New, Faster,Process for the Fabrication of Ceramic Fiber-Reinforced CeramicComposites by Chemical Vapor Infiltration,” Ceram. Eng. Sci. Proc., 6[7-8] 694-706 (1985).

Two U.S. patents have issued which involve a method for the fabricationof a fiber reinforced composite by combining an inorganic reinforcingfiber with dispersions of powdered ceramic matrix in organic vehicles,such as thermoplastics. The first patent, U.S. Pat. No. 5,024,978,discloses a method for making an organic thermoplastic vehiclecontaining ceramic powder that can form the matrix of a fiber reinforcedcomposite. This patent also discloses that the ceramicpowder/thermoplastic mixtures can be heated to above the melt transitiontemperature of the thermoplastic and then applied as a heated melt to aninorganic fiber. This patent further discloses that the process may beused to make composite ceramic articles. The second patent, U.S. Pat.No. 5,250,243, discloses a method for applying a dispersion of ceramicpowder in a wax-containing thermoplastic vehicle to an inorganic fiberreinforcement material to form a prepreg material such as a prepreg tow.This patent further discloses that the prepreg tow may be subjected to abinder pyrolysis step to partially remove the wax binder vehicle priorto consolidation of the prepreg tow into the preform of a compositeceramic article.

U.S. Pat. No. 5,936,861 discloses methods and apparatuses for makingthree-dimensional objects from continuous fiber reinforced compositematerials. Slurry infiltration techniques are used to create a “prepreg”of reinforcement fiber and matrix material. The prepreg is formed intothree-dimensional composite parts using a solid freeform fabricationprocess wherein the prepreg is extruded through a heated nozzle anddeposited onto a base member and solidified.

To summarize, the continuous fiber reinforced ceramic composites(“CFCCs”) prior to the present invention have traditionally beenfabricated using methods and apparatuses to infiltrate the matrix ormatrix-forming material around a preformed architecture of dense fibersor fiber tows or by passing the fibers through a powder/melt slurry.While these methods and apparatuses provide a fiber reinforced compositestructure, there is no control over the thickness of the matrix formingvehicle, and rarely will the matrix uniformly surround the fibers. Insuch methods, the fibers often contact each other which is detrimentalto the mechanical behavior of such composites. In addition, theseinfiltration processes are quite slow, sometimes requiring weeks ormonths to fabricate components, and are severely limited in thematrix/fiber combinations that can be produced.

Furthermore, currently available techniques for fabricating continuousfiber reinforced composite objects are not suited for mechanicallyforming fully dense objects having complex geometries from continuousfiber-reinforced filaments from ceramic powder raw materials.

Thus, there exists a need for more efficient methods and apparatuses forapplying the matrix to the fiber reinforcement. There exists a furtherneed for methods and apparatuses that are versatile enough to allowalmost limitless combinations of matrix and fiber reinforcement. Therealso exists a need for efficient methods and apparatuses for rapidlymaking three-dimensional objects, particularly objects having morecomplex geometries, from CFCCs, and particularly directly from computeraided designs (CAD).

SUMMARY OF THE INVENTION

The present invention comprises novel continuous composite coextrusionmethods and apparatus for fabricating fiber reinforced compositematerials, particularly two- and three-dimensional objects having morecomplex geometries. Specifically, the present invention comprises novelmethods and apparatus to fabricate composite materials via aneconomical, versatile, tow placement process that uses filaments ortapes produced by the controlled continuous composite coextrusionprocess. In a particular preferred embodiment of the present invention,multiple fiber tows (bundles of fibers) are introduced during meltextrusion of a ceramic (or metal)/binder feed-rod. The result of thiscoextrusion process is a coextruded “green” filament or tape containingan in-situ dense fiber or tow of fibers or multiple tows of fibers. Thefilament or tape is further processed or laid down using methods andapparatuses for rapid deposition of the green filament to provide two-and three-dimensional objects. Preferably, the objects are formed by anautomated process directly utilizing CAD drawings of such parts.

More specifically, the present invention relates to processes for therapid fabrication of a fiber reinforced composite, i.e., a compositewhich is comprised of a matrix of a material, such as a ceramic ormetallic material, and having fibers of a ceramic material dispersedwithin the matrix as a reinforcement with an interface to protect thefibers from degradation during fabrication and service. A preferredmethod of the present invention comprises: (a) forming a material-ladencomposition comprising a thermoplastic polymer and at least about 40volume % of a ceramic or metallic particulate in a manner such that thecomposition has a substantially cylindrical geometry and thus can beused as a substantially cylindrical feed rod; (b) forming a hole downthe symmetrical axis of the feed rod; (c) inserting the start of acontinuous spool of ceramic fiber, metal fiber or carbon fiber throughthe hole in the feed rod; (d) extruding the feed rod and fiberreinforcement simultaneously to form a continuous filament consisting ofa “green” matrix material completely surrounding a dense fiberreinforcement and said filament having an average diameter that is lessthan the average diameter of the feed rod; and (e) mechanicallydepositing the continuous filament into a desired architecture toprovide a green fiber reinforced composite. The green matrix may besubsequently fired, i.e., heated, to provide a fiber reinforcedcomposite with non-brittle failure characteristics.

The present invention also provides a process for the fabrication of afiber reinforced composite having an interlayer, i.e., a composite thatis comprised of a matrix of material, such as a ceramic or metallicmaterial, having fibers of a ceramic material dispersed within thematrix as a reinforcement, and having an interlayer that is between thematrix and fiber reinforcement. This method is the same as thatdescribed in the preceding paragraph, but further comprises forming afeed rod that contains two dissimilar particulate-laden compositionswherein during the extrusion process the second particulate-ladencomposition forms a green interlayer between the fiber reinforcement andthe green matrix in a continuous filament. This filament can be arrangedas described in the previous paragraph and both the green interlayer andthe green matrix may be subsequently fired to provide a fiber reinforcedcomposite having substantially improved non-brittle failurecharacteristics compared to a fiber reinforced composite in the absenceof an interlayer.

Additionally, the present invention also provides a process for theintroduction of multiple interlayers coated onto the fiber tows. Thismethod is the same as that described in the preceding paragraph, butalso includes forming a fiber tow coated with preceramic or carbonprecursors to form green interlayers during the extrusion processbetween the fiber reinforcement and the green matrix in a continuousfilament. This filament can be arranged as previously described, andboth the green interlayer and the green matrix may be subsequently firedto provide a fiber reinforced composite having substantially improvednon-brittle failure characteristics compared to a fiber reinforcedcomposite in the absence of an interlayer.

In a preferred method of the present invention, a co-axial filament isproduced with a fiber tow surrounded by a “green” ceramic. In a furtherpreferred embodiment of the present invention, the process has beendemonstrated utilizing carbon fiber tows in a hafnium carbide (“HfC”)matrix, a zirconium carbide (“ZrC”) matrix, and a silicon carbide(“SiC”) matrix. The resulting products can be used in extreme, hightemperature environments. The fibers impart the necessary thermal shockresistance and toughness that HfC, SiC and ZrC lack as monolithicceramics.

Further advantages of the present invention include the use ofinterlayers that can act to provide both non-brittle failurecharacteristics as well as oxidation protection during service. Multiplefiber tows and combinations of fiber types may be co-extruded to providecontinuous, flat, wide green ceramic tapes.

Another aspect of the present invention is the introduction of nanoparticulates of SiC, ZrC and alumina as sintering aids to the matrix.This assists in the reduction of the consolidation temperatures andresults in protection of the fibers during the consolidation stage.

The present invention further provides methods for the fabrication ofcontinuous filaments used in preparing fiber-reinforced compositeswherein the architecture of the filaments can be readily controlled.

Yet another aspect of the present invention is the ability to take thecontinuous filaments and form a shaped green-body, especially directlyfrom CAD designs. Generally, the continuous filament is passed to anapparatus for rapid deposition to form a two- or three-dimensionalobject of a desired shape and having a desired texture created by thearrangement of the filaments, such as based on the CAD design and thedesired fiber lay-ups. The continuous filament is mechanically depositedat a controlled rate onto a surface. The deposited material is heatedand subjected to compression forces to facilitate consolidation andbonding of the tow and substrate. The automated tow placement processjoins the fiber-reinforced filaments together, creating a sold, shapedgreen body. The process allows a wide range of body geometries to beformed, particularly more complex geometries, based on fiberorientations which can be determined from the desired mechanicalproperties of the final object such as specific loading conditions andby the CAD design

The processing techniques of the invention readily allows for control ofthe fiber volume fraction and changes to the matrix composition. Thistechnology is readily applicable to other matrix/fiber combinations andwill significantly enhance manufacturing capability for low cost,high-performance and high temperature ceramic composites. Objects formedin accordance with the compositions and methods of the invention aresuitable for use in high temperature and high performance applications,such as turbomachinery and propulsion applications, where enhancedablation and oxidation resistance at temperatures up to at least about3500° C., adequate load bearing capabilities, non-catastrophic failuremodes, and the ability to withstand transient thermal shock.

The present invention also contemplates that the complete process ofblending the matrix and interface materials, coating of the multipleinterface materials on the fiber or tow of fibers, co-extrusion of thefilament or tape and the rapid fabrication of the green composite partfrom the CAD design of the part can be integrated into a continuousprocess.

The processing techniques of the invention further allow the simulationof the fabrication of the 3-D object through a simulation program andanalytical modeling of the component mechanical properties to evaluatethe ability of the component to withstand the contemplated serviceconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-section of a preferred apparatus of thepresent invention.

FIG. 2 illustrates a cross-section of another preferred apparatus of thepresent invention.

FIGS. 3A and 3B are schematic illustrations of the matrix feedrodwithout and with the interface material feedrod, respectively, inaccordance with the present invention.

FIG. 4 is a flow chart illustrating a preferred method of the presentinvention.

FIG. 5A is a schematic illustration of a “green” coaxial filamentwithout an interface material and FIG. 5B is a schematic illustration ofa “green” coaxial filament with an interface material layer inaccordance with the present invention.

FIG. 6 is a schematic illustration of another preferred apparatus of thepresent invention illustrating its use as an automated tow placementprocess.

FIG. 7 is a graph illustrating viscosity and shear rate relationshipsfor compositions in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to rapid fabrication methods andapparatuses for making two- and three-dimensional objects fromcontinuous fiber reinforced ceramic, metal or intermetallic compositematerials. Generally, fiber reinforced filaments are prepared fromceramic powders and fiber tows. A fiber-reinforced feedrod isco-extruded with an interface or multiple interfaces and fiber tow ortows to provide a fiber reinforced “green” filament (single tow) or tape(two or more tows). The green filament or tape is fed through adeposition apparatus where heat and pressure are applied locally to thedeposited filament to form a two- or three-dimensional object of adesired shape and fiber arrangement. The object can be formed withdesired fiber orientations that can be determined from specific loadingconditions and CAD design of the objects. The apparatus is particularlysuited for making three-dimensional objects have more complex geometriesthan currently possible.

FIG. 1 illustrates a cross-section of a preferred apparatus for theco-extrusion of the matrix, interface and the fiber tow or tows offibers in the present invention. The apparatus 40 is an extrusion diecomprising an extrusion barrel 41, an extrusion ram 42, a cooling jacket53, a heating jacket 43, a transition block 44, a spinnerette 45, anextrusion orifice 46, and a motor driven winding spool 47, a motordriven ram screw 48, and an inlet 49.

FIG. 4 depicts a flow chart of a preferred method of the presentinvention. In accordance with a preferred method and apparatus of thepresent invention, as shown in FIGS. 1-3, a matrix feedrod 60 isprepared. A single component matrix feedrod 60 (FIGS. 2, 3A and 5A) canbe used. Preferably, one or more layers of interface material 50 areintroduced between the fiber tow 51 and the matrix feedrod 60 (FIGS. 1,3B and 5B) to limit the thermal expansion mismatch between the fiber andthe matrix during a subsequent consolidation step and to otherwise limitdamage to the filament during processing. The interface materialenhances composite strength and toughness by deflecting cracks. Acompound interface, such as where the fiber tow is coated with one ormore other interface materials prior to being inserted into the rod 50,can be used to further enhance mechanical properties and to enhance theoxidation resistance of the composite at elevated temperatures. Thecompound interface could be either solid, liquid or in paste (sol-gel)form. A feedrod having two or more longitudinal, generally parallelfiber tows similarly can be prepared.

The matrix feedrod 60 can be prepared by blending a suitable matrixfeedrod material, pressing the matrix feedrod material into a rod shape,and drilling a core hole 61 through the longitudinal axis of the matrixfeedrod 60. The interface material rod 50 also can be prepared byblending material effective for limiting thermal expansion mismatchbetween the fiber and the matrix feedrod and pressing the material intoa rod shape. The core hole 61 should have a diameter just large enoughfor the insertion of the interface material rod 50 there through. In apreferred embodiment, the matrix feedrod material comprises hafniumcarbide (“HfC”) or zirconium carbide (“ZrC”) or silicon carbide (“SiC”)and the interface material rod comprises graphite or boron nitride orsilicon carbide. Preferably the ceramic powder/thermoplastic blend ispressed into a “green” rod having a diameter of about 0.885 inches,i.e., about 2.248 cm.

The blending steps for the matrix feedrod 60 and interface material rod50 as shown in FIG. 1-3 can comprise milling, as necessary, and batchingof matrix feedrod and interface material powders individually withthermoplastic binders and additives. In preparing the material-ladencompounds used in the inventive methods, raw material powders aretypically blended with an organic polymer and, advantageously, one ormore processing aids.

The matrix feedrod is prepared using commercially available ceramic andmetal powders using a process for converting ordinary ceramic powderinto a “green” fiber that includes the powder, a thermoplastic polymerbinder and, advantageously, other processing aids. Mechanicallyactivated and agglomerate-free powders preferably are used. The powders,such as the metals, alloys, carbides, nitrides, oxynitrides,oxycarbides, borides, oxides, phosphates and silicides listed above, areselected to provide the desired mechanical properties in the finalcomposite, and a wide variety of combinations of powders may be used.Powders having particle size distributions in the range of about 0.01 toabout 100 microns (μm) in size may be used. Preferably, the particlesize of the powder is between about 1 to about 10 microns.

A wide variety of powder ceramics may also be used in the material-ladencompositions, affording a wide flexibility in the composition of thefinal fiber reinforced composite. Advantageously, powders that may beused in the first material-laden composition to provide the matrixfeedrod include diamond, graphite, ceramic oxides, ceramic carbides,ceramic nitrides, ceramic borides, ceramic silicides, metals, andintermetallics. Preferred powders for use in that composition includealuminum oxides, barium oxides, beryllium oxides, calcium oxides, cobaltoxides, chromium oxides, dysprosium oxides, neodymium iron borides, ironcobalt, and other rare earth oxides, hafnium oxides, lanthanum oxides,magnesium oxides, manganese oxides, niobium oxides, nickel oxides, tinoxides, aluminum phosphate, yttrium phosphate, lead oxides, leadtitanate, lead zirconate, silicon oxides and silicates, thorium oxides,titanium oxides and titanates, uranium oxides, yttrium oxides, yttriumaluminate, zirconium oxides and their alloys; boron carbides, ironcarbides, hafnium carbides, molybdenum carbides, silicon carbides,silicon oxycarbides, tantalum carbides, titanium carbides, uraniumcarbides, tungsten carbides, zirconium carbides; aluminum nitrides,aluminum oxynitrides, cubic boron nitrides, hexagonal boron nitrides,hafnium nitride, silicon nitrides, sialons, titanium nitrides, uraniumnitrides, yttrium nitrides, zirconium nitrides; aluminum boride, hafniumboride, molybdenum boride, titanium boride, zirconium boride; molybdenumdisilicide; lithium and other alkali metals and their alloys; magnesiumand other alkali earth metals and their alloys; titanium, iron, nickel,chromium, cobalt, molybdenum, tungsten, hafnium, rhenium, rhodium,niobium, tantalum, iridium, platinum, zirconium, palladium and othertransition metals and their alloys; cerium, ytterbium and other rareearth metals and their alloys; aluminum; carbon; lead; tin; and silicon.

Additionally, the matrix may be doped with oxidation-inhibitingadditives such as sodium silicate or boric acid or boron precursorresins to enhance the oxidation resistance of the composite material.The matrix may also contain additives such as nano-particulates of SiC,ZrC, HfC, Al₂O₃, Y₂O₃, or ZrO₂ and the like, in order to lower theconsolidation temperature of the composite system.

The interface material can include graphite, boron nitride, siliconcarbide, boron carbide, silicon nitride, sodium silicate, and boricoxide, either fully dense or in porous form. Additionally, to enhanceoxidation resistance of the matrix and fibers, sodium silicate powderscan be blended with the interface material. Anhydrous sodium silicatepowders such as SS 65 and SS-C 200 powders available from PQCorporation, Valley Forge, Pa., can be used.

Milling stations such as commercially available from Boston Gear,Boston, Mass. may be used as needed to ball mill the ceramic powder toobtain the desired size distribution. The desired ceramic powderpreferably is ball milled with ethanol. The ceramic/ethanol blend isball milled with milling media such as silicon nitride (Si₃N₄) orzirconium oxide (ZrO₂) thus creating a ball-mill slurry. Sintering aidssuch as, for example, aluminum oxide (Al₂O₃), silicon carbide (SiC) andyttrium oxide (Y₂O₃), when necessary, are added and milled together withthe ball mill slurry. The powders are milled for a time effective forproviding desired particle sizes and distribution. Typical milling timesare between about 24 to about 120 hours, depending on the startingceramic material. For example, boron nitride (BN) powder is milled forabout 12 to 24 hours, silicon nitride powder is milled for about 24hours, and zirconium carbide (ZrC), purchased as a fairly coarserefractory ceramic, is typically milled for a longer period, about 72 to120 hours.

Upon completion of the milling operation, the ball mill slurry iscollected from the milling station and the ceramic/ethanol mixture isseparated from the milling media using a perforated mill jar lid as a“strainer”. The ethanol is separated from the ceramic powder using aBuchi Rotavapor separator commercially available from BrinkmanInstruments Inc. of Westbury, N.Y. Solvent is evaporated from theball-milled slurry in the Buchi Rotavapor separator and the ceramicpowder dried. Ethanol solvent may be reclaimed as desired for reuse orproper disposal according to local, state, and federal waste disposalrequirements. The ceramic powders are removed from the separator jar andplaced in labeled plastic jars.

The individual powders for the matrix and interface material are blendedwith thermoplastic melt-spinnable polymer binders, as well as one ormore processing aids such as plasticizers as necessary, using a highshear mixer commercially available from C. W. Brabender of SouthHackensack, N.J. or from Thermo Haake of Paramus, N.J., to form asmooth, uniformly suspended composite blend also referred to as a“dope”. Examples of thermoplastic binders include polyethylene, ethyleneethylacetate (EEA) commercially available as DPDA-618NT from UnionCarbide, ethylene vinylacetate (EVA) commercially available as ELVAX 470from E.I. DuPont Co., and Acryloid Copolymer Resin (B-67) commerciallyavailable from Rohm and Haas, Philadelphia, Pa. Examples of plasticizersinclude heavy mineral oil (HMO) commercially available as Mineral OilWhite, Heavy, Labguard® and methoxy polyethyleneglycol having amolecular weight of about 550 (MPEG-550) commercially available fromUnion Carbide. The composite blend is compounded at about 150° C. whilemetering a viscosity-modifying additive until a viscosity is obtainedthat will ensure desired rheology for a molten fiber extrusion process.

After preparation of the matrix feedrod 60 and the interface materialrod 50, interface material rod 50 can then be inserted into and throughcore hole 61 of matrix feedrod 60. If desired, interface material rod 50and surrounding matrix feedrod 60 can then be repressed to maintaintheir rod shapes. A cylindrical hole 80 can next be drilled through thelongitudinal axis of interface material rod 50. In a preferredembodiment, cylindrical hole 80 has a diameter of about 0.125 inches,i.e., 0.318 cm.

The interface layer 50 surrounding the fiber tow 51, as shown in FIG.5B, has been found to reduce and eliminate matrix cracking in compositescaused by the large CTE mismatch between the matrix feedrod and fibermaterials. By pressing the graphite rods to different diameters, theinterface material layer 50 can be varied as desired.

The resulting combination of interface material rod 50 and surroundingmatrix feedrod 60 can then be inserted into inlet 49 and extrusionbarrel 41, until it stops at location 54. If desired, a guide tube canbe inserted through cylindrical hole 80 to facilitate feeding of thefiber tow through the hole.

Extrusion ram 42 can next be placed on top of the combination ofinterface material rod 50 and surrounding matrix feedrod 60. Extrusionram 42 has a bore 52 having a diameter of sufficient size to receive thefiber tow 51 and slide over the guide tube 20, if such a guide tube isused. If more than a single fiber tow is used, the diameter of the holesin the extrusion ram 42 is modified accordingly. For example, if four(4) fiber tows are used, the extrusion ram 42 is modified to have fourseparate holes to accept each individual fiber tow.

Prior to insertion into the feedrod 60, the fiber tow 51 can be coatedwith one or more materials to enhance the mechanical and oxidationinhibition properties of the composite. To provide increased bondingbetween the fiber and the matrix, the fiber can be coated with aphenolic resin to provide a porous graphite coating after sintering. Thephenolic coating can be used instead of the interface material layer, ifdesired. A catalyst, such as methyl-sulfonic acid catalyst, can be addedto the phenolic resin to facilitate rapid polymerization of the phenolicresin. The fiber also can be coated with platinum to provide oxidationresistance and to reduce thermal expansion mismatch. Preferably, thefiber is coated with platinum prior to being coated with the phenolicresin. Additionally, the fiber can be coated with sodium silicate toprovide a glassy interface that enhances oxidation resistance for thematrix and fiber. The individual fibers of the fiber tow can beseparated and passed through a bath to coat the fibers. The coating canbe rapidly cured with heating, and the fiber spooled before being passedto the extrusion apparatus 40. Additionally, the fiber tow or tows canbe impregnated with a coating of a refractory ceramic precursor resinover the phenolic coating that will result in the formation of SiC, HfC,ZrC, SiO₂, HfO₂, or ZrO₂ after pyrolysis. The fiber tows can also beimpregnated with a combination of ceramic precursor resin, boronprecursor and sodium silicate resulting in a borosilicate glassyinterface after pyrolysis. Additionally, an impregnated fiber tow, suchas an epoxy impregnated fiber tow can be used to impart the desiredcharacteristics to the final composite material.

Fiber tow 51 can then be inserted through bore 52 of extrusion ram 42and cylindrical hole 80 of interface material rod 50, until the insertedend reaches extrusion orifice 46. The dimensions of the orifice aredependent on the co-axial filament or tape size desired.

As shown in FIGS. 1-2, heating jacket 43 heats the matrix feedrod 60 tomelt the matrix feedrod material. Extrusion ram 42 pushes the matrixfeedrod 60 through hearing jacket 43 to the soften zone 56. Preferably,soften zone 56 has a frusto-conical shape, with the outlet 46 located atthe bottom of soften zone 56.

Co-axial filament or tape 70 is extruded from extrusion orifice 46 andwound on the motor driven spinnerette or winding spool 47. The coolingjacket 53 is used to lower the temperature along the extrusion barrel 41away from the heating jacket 43, so that only the extrusion end of thebarrel 41 is maintained at the extrusion temperature. Maintaining alower temperature along the top end of the barrel 41 allows theintroduction of the fiber tow as late as possible in the extrusionprocess to reduce binding failures and lessen the tension required topull the fiber tow through the feedrod. Co-axial filament 70 thuscomprises fiber tow 51, surrounded by interface material rod 50 andmatrix feedrod 60. Co-axial filament 70 can also be called a greenZrC/C_(f) filament, if ZrC is used as the matrix feedrod material, andthe tow comprises a carbon fiber material.

Notably, the fiber tow 51 is centered in the green co-axial filament 70.If multiple tows used, they preferably are spaced equidistant from theedge of the co-axial filament. Design choices to achieve the desiredproduct include varying the viscosities of ceramic powder/thermoplastic,interface material powder/thermoplastic blends or the ceramic precursorresin or sol-gel, and changing the composite fiber extrusion conditions.These choices can lead to a uniform interfacial coating.

A wide variety of fibers can be used in accordance with the presentinvention. The type of fiber to use is a design choice, as are types offiber tows. For example, ceramic fibers can comprise silicon carbide orgraphite/carbonmetal fibers can comprise tungsten, tantalum, steel,aluminum, and copper fibers, and magnetic fibers can comprise nickel,cobalt and tungsten wire. In choosing a fiber tow, factors to considerinclude fiber tow diameter, tow strength, tow elastic modulus, and thecoefficient of thermal expansion (CTE). Three examples of carbon fibersthat can be used in accordance with the present invention are as followsin Table 1. TABLE 1 Carbon Fiber Tow Properties Tow Tensile TensileDiameter Strength Modulus Axial CTE Supplier Fiber Type (mm) Gpa (ksi)Gpa (Msi) ppm/K Hexcel AS4 3K 0.387 5.93 (570) 221 (32) −0.7 HexcelUHMS-G 3K 0.242 3.48 (500) 441 (64) −0.5 Amoco T-300 3K 0.393 3.65 (530)231 (33.5) −0.6

The above fibers from Hexcel and Amoco are termed polyacrylonitrile(“PAN”) type of fibers. Fibers from Amoco derived from a petroleumextract, referred to as “pitch” and commercially sold by Amoco with aprefix “P” could also be used. To improve fiber volume fraction,multiple fiber tows can be used. By way of example, four 3K-sized can beused to provide a 12K fiber tow in the filament after extrusion, andfiber volume fraction is about 25 to about 35%.

The starting fiber tow diameter is a factor in determining the fibervolume fraction of final composite parts. The tow strength and towstiffness governs mechanical properties such as flexural and tensilestrength in the final composite. The CTE of the fiber will determine thecompatibility of the fiber/matrix and the size/type of interface. Forexample, the reported CTE value of the ZrC matrix is 6.9 ppm/K, whileaxial CTE of carbon fiber is less than 0 ppm/K. In order to minimizethis CTE mismatch, a graphite interfacial coating is placed between thecarbon fiber and ZrC matrix during co-extrusion.

After coextrusion, the fiber-reinforced filament can be passed through abath of butyl oleate or the like to increase filament flexibility. Forexample, the filament 70 can be passed to a bath of heated butyl oleatesubsequent to extrusion at the orifice 46. The filament 70 passesthrough and is immersed in the bath for a time sufficient to coat thefilament before it is wound on the spool 47.

Although extrusion occurs generally as a batch process, it is possibleto prepare a continuous filament having any desired length, as long asthe end of the filament is not passed through the orifice 46 prior toextrusion of another feedrod. In this way, a continuous filament can beprepared and wound on the spool 47. The spooled filament can be helduntil it is subsequently formed into the desired object or can be passeddirectly apparatus for forming the object.

In other embodiments, the filament can be prepared by any modifiedcontinuous coextrusion process. As an example, the raw matrix andinterface materials can be fed directly to an extrusion mechanism, alongwith the fiber tow, and extruded to form a continuous filament.

In other embodiments, the filament can be prepared from liquid precursormaterials using any modified slurry infiltration process.

The prepared filament 70 is further processed using a method andapparatus for automated lay up of the filament to mechanically form atwo- or three-dimensional object having a desired shape and filamentarchitecture. The filament is deposited layer-by-layer in apredetermined arrangement to form the object. The filament can bedeposited onto a flat surface, around a mandrel, or other suitablesurface. Concurrent use of localized heat and pressure is utilized informing the objects so that bonding is achieved between the newlydeposited filament and the already-deposited composite material and alsoto consolidate the deposited composite material. The layer-wiseformation is dependent on the fiber orientations which can be determinedfrom specific loading conditions of the desired object and CAD design ofthe object.

Referring now to FIG. 6, there is illustrated a preferred apparatus 100for forming fiber reinforced composite materials. Generally, theapparatus 100 includes a filament feed assembly 102 through which thefilament 70 is passed from the spool 47, one or more heat sources 104,106, one or more compression mechanisms 108, 110, and a working surface112. Preferably, the heat sources include a hot-gas nitrogen torch suchas described in U.S. Pat. No. 5,626,471 which is incorporated byreference herein in its entirety. Preferably the compression mechanismsare rollers.

The filament 70 is directed by the feed assembly 102 and laid up ontothe working surface 112 in layers 116. The heat sources 104, 106 areused to heat the filament as it is deposited on the working surface 112,as well as to heat the surface 114 of the laid up filament compositematerial. The first heat source 104 preheats the composite surface 114adjacent the location where the filament is deposited, along with theincoming filament tow 70. The first roller 108 is positioned behind thepoint of filament deposition 118 in the direction of movement 122 of theapparatus and provides a compression force to the deposited filament andcomposite layers 116. The filament material 70 is thus “tacked” to thesurface 114 by the first roller 108. This tacking procedure is useful inthat the fed material is carefully bonded to the surface 114 and notpulled with the main consolidation roller 110. This tacking approachalso aids in improving the efficiency of the cut and refeed mechanism(not shown). The second, main heat source 106 provides supplementalheating through substantially the thickness of the laid up material 116to facilitate consolidation and bonding of the tow 70 and substrate 114under the consolidation roller 110. The rollers 108, 110 are designed toprovide the necessary forces to achieve intimate contact and bondingacross the newly-formed tow interface for lamination and to achieveconsolidation within the materials, as well as to provide boundarypressure to limit development of internal voids.

Overlap between adjacent layers is necessary so that no porosity existsin the final object. When pressure is applied during placement of thefilament, the filament will be compressed and its dimensions willchange. For example, a 1 mm high by 2 mm wide filament can be compressedto less than 1 mm by more than 2 mm. The extent to which the dimensionschange is dependent on the temperature and pressure of the system.

The forces applied to both rollers 108, 110 are controlled independentlyusing a series of pneumatic actuators. The composite tow layers 116 canbe placed in a regular repeating pattern or with brick-face symmetry120, as shown in the enlarged section of FIG. 10. The brick-facegeometry has the advantage that more homogeneity is achieved throughoutthe composite structure. All the processing inputs are controlled eithermanually or through a PID control scheme from a LabVIEW™ interface.

The motion of the apparatus 110 and process parameters in forming thecomposite object preferably are computer-controlled. Preferably, arobotic work cell, such as an ABB IRB 6400 robotic work cell from AseaBrown Boveri (ABB) of Norwich, Conn. is used to control the motion ofthe apparatus. Green ceramic matrix laminates of any size (within limitsof the robotic work cell), fiber orientation and material system can befabricated in accordance with the methods of the invention.

Parameters including heat source temperature, height and gas flow rate,consolidation force and head velocity can be controlled as inputs tocomputer software. Additionally, final panel dimensions and lay-upsequences preferably based on CAD drawings of the desired object areinputs to the program. A computer-controlled nitrogen hot gas torchcontrol system is used to monitor and control gas flow rate andtemperatures within both heat sources 104, 106. The composite surface114 temperatures can be adjusted by either increasing the processvelocity or by independently adjusting the height of the heat sources.Temperature can be measured and controlled with an AGEMA Thermal Imagingcamera with a neural network based PID control system. The camerameasures the viewable peak temperatures on the laminate surface andadjusts the heat source heights accordingly to compensate for anydeviation in set point temperatures. A Labview™ interface is used toinput number of layers, ply orientation, surface temperatures, panelgeometry, and process velocity. This interface can also be used inconjunction with a laser displacement unit to measure warpage duringprocessing.

Typical process parameters for ceramic tow placement are listed in Table2. TABLE 2 Sample Process Parameters for Ceramic Tow Placement ProcessParameter Value Initial thickness of tow 1.0 mm Width of tow 2.0 mmRadius of first roller 15.8 mm Radius of second roller 19.0 mm Secondroller location 80 mm from first roller Gas Flow rate in torches 50liters/min Location of first torch 75 mm from nip point locationLocation of main torch 35 mm from nip point Torch temperatures 850° C.Torch heights 12 mm Head velocity 50 mm/s Consolidation force 190 N

These process parameters can be adjusted according to the apparatus usedand the properties of the composite materials. For example, thetemperature of the first heat source can be adjusted from between about850° C. to about 500° C., preferably about 500° C. In addition, in orderto prevent the matrix from adhering to the rollers, the rollers requireactive cooling so that the roller temperature does not exceed the melttemperature of the binder. This is accomplished by positioning the mainheater torch 106 to actively cool the second roller 110. For example,the gas flow rate for first heat source 104 is about 50 liters/min. andfor the main heat source 106 is about 25 liters/min. Additionally, thefirst heat source 104 and/or first roller 118 can be disabled if theproperties of the filament are such that the filament is too soft at thetemperature of deposition so that the pressure applied by the firstroller 118 would cause the matrix material to be squeezed out under theroller. The main heat source 106 can be positioned above the secondroller 110 to actively cool it and maintain its temperature below theglass transition temperature of the filament material to prevent thematrix of the tows from adhering to the roller 110 and creating barefiber spots and inconsistent quality.

The composite object can then be placed into a furnace and subjected toheat to burn out any remaining thermoplastic binder. Further, thecomposite object can be consolidated using any suitable method,including but not limited to, hot pressing, hot isostatic pressing,pressureless sintering, and self propagating high temperature synthesis,all of which are known to those skilled in the art. The consolidationstep forms a fully dense fiber reinforced composite material. Theresulting product of these steps is a fiber reinforced composite object.

The continuous composite coextrusion process of the present inventioncan be used for ceramic matrix composites (“CMCs”) and metal matrixcomposites (“MMCs”). Further, the use of interlayers helps to controlstresses due to mismatches among the coefficients of thermal expansion(“CTE”), including those set forth above. Further, the present inventionreduces microcracking. In addition, the self-propagating, hightemperature synthesis is versatile, although it requires an additionaldensification step.

The process of the present invention can be accomplished using varioussuitable materials, such as ceramic powders (having different particlesizes), thermoplastics, and plasticizers. The present invention can alsoincorporate various modifications to various steps, including the stepsof compounding, making feed rods, passing the fiber/fiber tow throughthe feed rod, and using spinnerettes for extrusion. Further, the presentinvention can be used to achieve more than one fiber tow and/or morethan one coating on a fiber/fiber tow (interlayers), and that the coatedfibers/fiber tows of the present invention can be used to form variousfiber reinforced ceramic objects.

EXAMPLES

The following examples further illustrate preferred embodiments of thepresent invention but are not be construed as in any way limiting thescope of the present invention as set forth in the appended claims.

Example 1

Hafnium Carbide Matrix/No Interface/Carbon Fiber Reinforcement

VPCA-BR00

-   Description: Core Material-   Brabender Size: small-   Batch Size: 42 cc-   Batch Temperature: 150° C.-   Batch Speed: 60 rpm

Ingredients: TABLE 3 Material Density (g/cc) Volume % Volume (cc) Weight(g) HfC 12.67 54.0% 22.66 287.36 EEA 0.93 32.4% 13.608 12.66 B-67 0.943.6% 1.512 1.42 HMO 0.881 10.0% 4.2 3.70

In the above-cited formulation of Table 3, HfC is hafnium carbide powderfrom Cerac, Inc., designated as H-1004, B-67 is acryloid resin from Rohmand Haas, EEA is ethylene-ethyl acetate copolymers, and HMO is heavymineral oil which is a plasticizer. A “Brabender” mixing machine (fromC. W. Brabender of South Hackensack, N.J.) was used to mix theabove-cited materials. The mixture of materials can then be formed intoa feed rod with a hole through the symmetrical axis of the feed rod.After mixing, the mixture was formed into a feed rod-like shape likethat shown in FIG. 1 and in detail in FIG. 3B. The carbon fiberreinforcement can be inserted into the hole of the matrix as desired.Following coextrusion, the result is a “green” material that stillcontains binder, like that shown in FIG. 5. This green material can nowbe formed in a desired manner, such as a billet. The billet can then besubjected to lamination in a warm pressing operation to fill remainingvoids, and the result is a green billet. The green billet can then besubjected to pyrolysis and then the resulting part can be hot pressed,hot isostatic pressed, or pressureless sintered to densify the matrix.

Example 2

Hafnium Carbide Matrix/Graphite Interface/Carbon Fiber Reinforcement

The hafnium carbide matrix made in accordance with Example 1 is the samematrix for Example 2. The only difference in Example 2 is that the holethrough the symmetrical axis of the feed rod is made larger so that agraphite interface can be inserted through the hole of the feed rod. Thegraphite interface defines a hole through its symmetrical axis, and thecarbon fiber reinforcement can be inserted into the hole of the graphiteinterface, resulting in the product illustrated in FIG. 3B. Followingcoextrusion, desired formation (such as a billet), lamination,pyrolysis, and firing as described in Example 1 and 2 the result is afully dense composite formation. The formulation for the graphiteinterface is as follows.

VPCA-BR06

-   Description: Core Material-   Brabender Size: small-   Batch Size: 42 cc-   Batch Temperature: 150° C.-   Batch Speed: 60 rpm

Ingredients: TABLE 4 Material Density (g/cc) Volume % Volume (cc) Weight(g) Graphite-4929 2.25 49.0% 113.19 254.68 EEA 0.93 49.0% 113.19 105.27MPEG-550 1.104 2.0% 4.62 5.10

In the above formation of Table 4, MPEG-550 is methoxy polyethyleneglycol 550 (i.e., having an average molecular weight of 550). Aspreviously noted, graphite interface has a hole through its symmetricalaxis so that the carbon fiber reinforcement can be inserted through thataxis as desired.

Various grades of materials can be used in accordance with the presentinvention, including various grades of HfC and graphite.

Example 3

The present invention can be used to make other reinforcements,including but not limited to:

-   -   Zirconium Carbide Matrix/Graphite Interface/Carbon Fiber        Reinforcement;    -   Zirconium Carbide Matrix/No Interface/Carbon Fiber Reinforcement        or Silicon Carbide Reinforcement;    -   Silicon Carbide Matrix/No Interface/Carbon Fiber Reinforcement;    -   Hafnium Diboride Matrix/Graphite Interface/Carbon Fiber        Reinforcement;    -   Silicon Carbide Matrix/Boron Nitride Interface/Silicon Carbide        Reinforcement;    -   Silicon Nitride Matrix/Boron Nitride Interface/Silicon Carbide        Reinforcement;    -   Silicon Carbide Matrix/Boron Nitride-Sodium Silicate        Interface/Carbon Fiber Reinforcement;    -   Silicon Carbide Matrix/Carbon-Silicon Carbide Interface doped        with boron/Carbon Fiber Reinforcement;    -   Silicon Carbide Matrix/Carbon-Silicon Carbide-Sodium Silicate        Interface doped with boron/Silicon Carbide Fiber Reinforcement;    -   Silicon Carbide Matrix/Boron Nitride-Sodium Silicate        Interface/Silicon Carbide Fiber Reinforcement;    -   Silicon Carbide Matrix/Carbon-Silicon Carbide Interface doped        with boron/Silicon Carbide Fiber Reinforcement; and    -   Silicon Carbide Matrix/Carbon-Silicon Carbide-Sodium Silicate        Interface doped with boron/Silicon Carbide Fiber Reinforcement.

The continuous composite coextrusion process of the present inventioncan be used to make a wide range of products having varying solidsvolume percentages and varying fiber diameters, such as the followinghafnium carbide matrix (“HfC”) and C_(f) (“carbon fiber reinforcement”)products:

1. HfC/C_(f) (25 vol. %), 18 μm

-   -   carbon black interlayer

2. HfC/C_(f) (25 vol. %), 32 μm

-   -   carbon black interlayer.

3. HfC/C_(f) (12.5 vol. %), 45 μm

carbon black interlayer. Thermal Expansion Considerations Material CTE(×10⁻⁶ K⁻¹) C_(f) −0.1 (axial) HfC 7.2-8.2* TaC 7.3 HfB₂ 7.9 ZrB₂ 8.2SiC 5.8*Coors Analytical Laboratory

Example 4

ZrC may be pressureless sintered using sintering additives, for example,zirconium metal. The following examples show the density and flexuralstrength of composites wherein the consolidation was accomplished bypressureless sintering.

NCE-BR01

-   Description: Core Material-   Brabender Size: small-   Batch Size: 42 cc-   Batch Temperature: 150° C.-   Batch Speed: 60 rpm

Ingredients: TABLE 5 Material Density (g/cc) Volume % Volume (cc) Weight(g) ZrC (10% SiC) 6.35 53.65% 22.53 143.08 EEA 0.93 30.00% 12.60 11.72Wax 0.92 3.75% 1.58 1.45 B-67 1.06 5.27% 2.23 2.35 Butyl Oleate 0.877.33% 3.09 2.69

NCE-BR02

-   Description: Graphite Interlayer Material-   Brabender Size: small-   Batch Size: 42 cc-   Batch Temperature: 150° C.-   Batch Speed: 60 rpm

Ingredients: TABLE 6 Material Density (g/cc) Volume % Volume (cc) Weight(g) Graphite 1.80 53.65% 22.53 37.04 EEA 0.93 30.00% 12.60 11.72 Wax0.92 6.75% 2.84 2.61 B-67 1.06 5.27% 2.23 2.35 Butyl Oleate 0.87 8.98%3.78 3.29

Thermal stresses and associated fractures were reduced in the productionof relatively crack-free ZrC composites. Further reduction of thermalstresses and degradation of the carbon fibers was achieved duringconsolidation. This was accomplished by using Hexcel UHMS-G carbon fibertow. It is believed that the higher elastic modulus of this fiber wouldhelp reduce the clamping forces on the fibers produced by the CTEmismatch and thereby eliminate microcracks. In addition, the fiberarchitecture was varied to better distribute the residual stresses. Twobillets were prepared using ZrC (10 vol % SiC) powder.

Example 5

Feedrods were produced from the ZrC preliminary formulation consistingof 10 vol % nano-SiC as a sintering aid. Feedrods were also producedfrom BN powder. Blend formulations for the interface BN were developedusing similar thermoplastic polymer binders as the ZrC to achieveconsistent co-extrusion. Details of compounding recipes are given inTables 7 and 8. TABLE 7 Type ZrC with 10 vol % nano-SiC Batch Size 231cc Batching Temp. 150 deg C. Batching Speed 60 rpm Material Density(g/cc) Volume % Volume (cc) weight (g) ZrC 6.35 53.65 123.93 787.09 EEA(MFI 20) 0.93 30.00 69.30 64.45 Luwax Al3 0.92 3.75 8.66 7.97 B-67 1.065.27 12.17 12.90 Butyloleate 0.87 7.33 16.93 14.73 Totals 3.84 100.00231.00 887.14

TABLE 8 Type BN Interface Material Batch Size 42 cc Batching Temp. 150deg C. Batching Speed 60 rpm Material Density (g/cc) Volume % Volume(cc) weight (g) Boron Nitride 2.250 49.00 20.58 46.31 B-67 1.060 5.272.2134 2.35 Al3 Wax 0.920 6.75 2.835 2.61 EEA (MFI 20) 0.930 30.00 12.611.72 Butyl Oleate 0.873 8.98 3.7716 3.29 1.578 100.00 42.00 66.27

The compounded material was pressed into rods (22.22 mm dia.×about127.00 mm length) in a stainless steel die at 140° C. for 5 minutes.Composite feedrods were created then extruded with the phenolic-coatedfibers into continuous “green” composite filaments. Extrusion of theserods was carried out at 140° C. and an extrusion rate of about 3.00cc/min.

Example 6

Sintering aid additives that will yield samples having sufficientdensities at lower consolidation pressure and temperature conditionswere tested. The ZrC core material was blended with 10-vol % nano-SiC asa sintering aid. The need for lower consolidation temperatures isrelated to maintaining the post-consolidated integrity and mechanicalproperties of the reinforcing carbon fiber tows. If the finalconsolidation temperature of the composite is higher than thetemperature to process the tows, fiber integrity will be destroyedduring consolidation process. Therefore, it is important to determineeffective sintering aids that will counteract the potential forlow-density parts sintered under the necessary reduced pressure andtemperature conditions.

For consistent co-extrusion of the core and interface materials, therheology of the ZrC should closely match the rheology of the BNinterface material (the BN was formulated using the same binder systemas the ZrC). The initial ZrC/binder system with 53.65 vol % solidsloading showed inconsistent BN interfacial coating along the length ofthe extruded filament, possibly due to the higher viscosity created dueto the nano-SiC addition. Therefore, the rheology of the ZrC core and BNinterface formulations were evaluated using the Instron capillaryrheometer. Rheology results on the ZrC blend showed the formulation wasmis-matched to the BN by nearly an order of magnitude. The vol % solidsloading of the ZrC was then adjusted to 50-vol %, to match that of theBN formulation. Rheology results on the new ZrC formulation show a goodmatch of the two blends (FIG. 7). Co-extrusion of the 50% ZrC and BN at150° C. yielded filaments with a consistent BN interfacial layerthickness.

Example 7

Two green laminates were fabricated with ceramic tows: (1) 225 mmlong×75 mm wide [0/0] (9″ long×3″ wide); and (2) 150 mm long×75 mm wide[0/90/0] (6″ long×3″ wide). For both cases, the process parameters arelisted in Table 9. For both samples, the entire robot movement sequencewas setup by computer programs. TABLE 9 Process Parameter Value Initialthickness of tow 1.0 mm Width of tow 2.0 mm Radius of first rollerDisabled Radius of second roller 19.0 mm Second roller location 80 mmfrom first roller Gas Flow rate in first torch 50 liters/min Gas Flowrate in main torch 25 liters/min Location of first torch 75 mm from nippoint location Location of main torch Above second roller First torchtemperature 500° C. Main torch temperature 25° C. First torch heightMaximum (12 mm) Head velocity 50 mm/s Consolidation force Minimum (190N)

Many modifications and variations may be made in the techniques andstructures described and illustrated herein without departing from thespirit and scope of the present invention. Accordingly, the techniquesand structures described and illustrated herein should be understood tobe illustrative only and not limiting upon the scope of the presentinvention.

1. A composite object formed by the steps comprising: (a) forming acontinuous filament comprising a longitudinally extending continuousfiber and a material-laden composition comprising a thermoplasticpolymer and at least about 40 volume % of a ceramic or metallicparticulate, wherein the filament includes a green matrix material fromthe material-laden composition, and wherein the green matrix materialcompletely surrounds the fiber; (b) passing the filament to a movableassembly for guiding placement of the filament onto an associatedworking surface; (c) depositing the filament from the movable assemblywithout application of a compression force onto the working surface toform a lower filament layer having a predetermined filament orientation;(d) depositing the filament from the movable assembly withoutapplication of a compression force onto the working surface to form anupper filament layer on top of the lower filament layer; (e) heating thefilament after it is deposited in the upper filament layer, a portion ofthe upper filament layer adjacent the deposited filament and a portionof the filament layer below and proximate to the deposited filament to apredetermined temperature effective for softening the green matrixmaterial to provide a heated portion of deposited filament and filamentlayers; (f) compressing the heated portion with a force effective forconsolidating and bonding the green matrix material of the depositedfilament and filament layers; (g) solidifying the heated portion; (h)repeating steps (d)-(g) one or more times as desired to provide acomposite object comprising two or more filament layers and having apredetermined geometry.
 2. The composite object of claim 1 formed by thefurther step of preheating the filament as it is deposited onto the worksurface to a temperature effective for adhering the filament topreviously deposited filament.
 3. The composite object of claim 1wherein the filament includes one or more interface layers between thematrix material and the fiber for enhancing non-brittle failurecharacteristics of the composite and oxidation protection.
 4. Thecomposite object of claim 3 wherein the one or more interface layersinclude materials selected from the group consisting of graphite, boronnitride, silicon carbide, boron carbide, silicon nitride and blendsthereof.
 5. The composite object of claim 1 wherein the filamentincludes a plurality of discrete fibers.
 6. The composite object ofclaim 1 formed by the further step of immersing the filament in acomposition effective for increasing flexibility of the filament priorto depositing the filament onto the working surface.
 7. The compositeobject of claim 1 formed by the further steps of: (a) creating a drawingof the desired composite three-dimensional object utilizing acomputer-aided design process, wherein the process generates a drawingincluding a plurality of segments; and (b) generating input signalsbased on the drawing for directing the movable assembly in thedepositing the filament onto the working surface, wherein the movableassembly is guided in response to the signals.
 8. The composite objectof claim 1 formed by the further steps of blending a thermoplasticbinder with the material-laden composition and heating the compositeobject to remove thermoplastic binder from the composite object andconsolidating the composite object to provide a fully dense fiberreinforced composite object.
 9. The composite object of claim 1, whereinthe composite object is heated to a temperature and for a time effectivefor sintering the green matrix material.
 10. The composite object ofclaim 1, wherein the filament is cut after a length of filament has beendeposited on the working surface.
 11. The composite object of claim 1,wherein the compression force is applied using one or more rollers. 12.The composite object of claim 1, wherein the heated portion iscompressed with a force of about 190 newtons.
 13. A three-dimensionalcomposite object formed by the steps comprising: (a) forming a feed rodhaving a longitudinal axis and having a hole extending down thelongitudinal axis, the feed rod comprising a material-laden polymercomposition comprising a thermoplastic polymer and at least about 40volume % of a ceramic or metallic particulate; (b) inserting an end ofone or more fibers through the hole in the feed rod; (c) extruding thefeed rod and one or more fibers simultaneously to form a continuousfilament that includes a green matrix material from the material-ladencomposition, the filament having an average diameter that is less thanthe average diameter of the feed rod and the green matrix materialcompletely surrounding the fiber; (d) passing the filament to a movableassembly for guiding placement of the filament onto an associatedworking surface; (e) depositing the filament from the movable assemblywithout application of a compression force onto the working surface toform a lower filament layer having a predetermined filament orientation;(f) depositing the filament from the movable assembly withoutapplication of a compression force onto the working surface to form anupper filament layer on top of the lower filament layer; (g) heating thefilament after it is deposited in the upper filament layer, a portion ofthe upper filament layer adjacent the deposited filament and a portionof the filament layer below and proximate to the deposited filament to apredetermined temperature effective for softening the green matrixmaterial to provide a heated portion of deposited filament and filamentlayers; (h) compressing the heated portion with a force effective forconsolidating and bonding the green matrix material of the depositedfilament and filament layers; (i) solidifying the heated portion; (j)repeating steps (f)-(i) one or more times as desired to provide acomposite object comprising two or more filament layers and having apredetermined geometry.
 14. The composite object of claim 13, whereinthe composite object is heated to a temperature and for a time effectivefor sintering the green matrix material.
 15. The composite object ofclaim 13, wherein the filament is cut after a length of filament hasbeen deposited on the working surface.
 16. The composite object of claim13, wherein one or more rollers compress the heated portion.
 17. Thecomposite object of claim 13, wherein the heated portion is compressedwith a force of about 190 newtons.